Silver halide photothermographic imaging materials, especially "dry silver" compositions, processed with heat and without liquid development have been known in the art for many years. Such materials are a mixture of light insensitive silver salt of an organic acid (e.g. silver behenate), a minor amount of catalytic light sensitive silver halide, and a reducing agent for the silver source.
The light sensitive silver halide is in catalytic proximity to the light insensitive silver salt such that the latent image formed by the irradiation of the silver halide serves as a catalyst nucleus for the oxidation-reduction reaction of the organic silver salt with the reducing agent when heated above 80.degree. C. Such media are described in U.S. Pat. Nos. 3,457,075; 3,839,049; and 4,260,667. Toning agents can be incorporated to improve the color of the silver image of photothermographic emulsions as described in U.S. Pat. Nos. 3,846,136; 3,994,732 and 4,021,249. Various methods to produce dye images and multicolor images with photographic color couplers and leuco dyes are well known in the art as represented by U.S. Pat. Nos. 4,022,617; 3,531,286; 3,180,731; 3,761,270; 4,460,681; 4,883,747 and Research Disclosure 29963.
A common problem that exists with these photothermographic systems is the instability of the image following processing. The photoactive silver halide still present in the developed image may continue to catalyze print-out of metallic silver even during room light handling. Thus, there exists a need for stabilization of the unreacted silver halide with the addition of separate post-processing image stabilizers or stabilizer precursors to provide the desired post-processing stability. Most often these are sulfur containing compounds such as mercaptans, thiones, thioethers and development inhibitor releasing compounds as described in Research Disclosure 17029 and U.S. Pat No. 3,700,457. Examples of stabilizer precursors in photothermographic materials are described in U.S. Pat. Nos. 3,839,041 and 3,301,678. U.S. Pat. Nos. 4,351,896 and 4,404,390 describe the use of blocked mesoionic 1,2,4-triazolium-3-thiolates as silver halide stabilizer precursors in which the sulfur atom is blocked by an appropriate blocking group which is cleaved upon processing at processing temperatures to provide a moiety that combines with the photoactive silver halide in the unexposed and undeveloped areas of the photographic material. The resulting silver mercaptide is more stable than silver halide to light, atmospheric and ambient conditions. However, one of the problems with stabilizer precursors is the inadequate release of the stabilizing moiety within the desired time frame during processing.
Specifically, in connection with this invention, U.S. Pat. No. 4,245,033 describes sulfur compounds of the mercapto-type that are development restrainers of photothermographic systems. The use of substituted 5-mercapto-1,2,4-triazoles with immobilizing groups that are of a ballasting polymer type or hydrophilic in nature such as sulfo, hydroxyl, carboxyl or sulfinic acid as development restrainers are also described in U.S. Pat. No. 4,837,141. Mesoionic 1,2,4-triazolium-3-thiolates as fixing agents and silver halide stabilizers are described in U.S. Pat. No. 4,378,424. Substituted 5-mercapto-1,2,4-triazoles such as 3-amino-5-benzothio-1,2,4-triazole as post-processing stabilizers are described in U.S. Pat. No. 4,128,557; 4,137,079; 4,138,265, and Research Disclosures 16977 and 16979. U.S. Pat. No. 4,451,561 describes amido derivatives of 5-mercapto-1,2,4-triazoles as development restrainers.
Some of the problems with these stabilizers include thermal fogging during processing or losses in photographic sensitivity, maximum density or contrast at stabilizer concentrations in which stabilization of the post-processed image can occur. Thus, there has been a continued need for improved post-processing stabilizers which stabilize the photoactive silver halide for post-processing stabilization without desensitizing or fogging the photographic materials.